금요일, 9월 17
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#028 J2SE 5.0 Annotations

Lab Exercises

 

Exercise 1: Override annotation

Programmers sometimes overload a method when they mean to override it; a classic example of this is the equals() method which is defined in the Object class.  AnnotatedClass.java contains a simple example of this:

public boolean equals(String otherName) {

int comparisson = name.compareTo(otherName);
return (comparisson == 0);

}

The programmer thinks that they are overriding the equals() method from the Object class, but since that takes an Object as a parameter the result (which is perfectly correct Java) is an overloaded method instead.  This could, potentially cause some very subtle and therefore hard to debug runtime errors.  Adding a standard annotation type to this can identify these problems at compile time.

(1.1) Override annotation

0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in OverrideAnnotation as project name.
  • For Create Main Class field, type in OverrideAnnotation.
  • Click Finish.

  • Observe that OverrideAnnotation project appears and IDE generated OverrideAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated OverrideAnnotation.java as shown in Code-1.11 below.  Study the code by paying special attention to the bold fonted parts.

public class OverrideAnnotation {

private String name;
private int id;

/**
*  Constructor
**/
public OverrideAnnotation() {
name = “Java Passion!”;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding vs. overloading
*  For exercise 1 change this method
**/
public boolean equals(String otherName) {
int comparisson = name.compareTo(otherName);

return (comparisson == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-1.11:  equals() method without @Override annotation

3. Build and run the project

  • Right click ExtendThreadClassTest0 project and select Run.
  • Observe the compilation of the code succeeds. (Figure-1.13 below) This is not desirable.  What we want is compile time detection of a problem.
run:
BUILD SUCCESSFUL (total time: 1 second)

Figure-1.13: Result of running ExtendThreadClassTest0 application

5. Modify the ExtendThreadClassTest0.java as shown in Code-1.15 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class OverrideAnnotation {

private String name;
private int id;

/**
*  Constructor
**/
public OverrideAnnotation() {
name = “Java Passion!”;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding vs. overloading
*  For exercise 1 change this method
**/
@Override
public boolean equals(String otherName) {
int comparisson = name.compareTo(otherName);

return (comparisson == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-1.15: equals() method with @Override annotation

6. Observe that the compiler now detects a problem.  This is a desired behavior.


Figure-1.16: Result

7. Modify the ExtendThreadClassTest0.java as shown in Code-1.17 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class OverrideAnnotation {

private String name;
private int id;

/**
*  Constructor
**/
public OverrideAnnotation() {
name = “Java Passion!”;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding vs. overloading
*  For exercise 1 change this method
**/
@Override
public boolean equals(Object otherName) {
String newName = (String) otherName;
int comparison = name.compareTo(newName);

return (comparison == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-1.17: Make it overriden method

8. Build the project.  Observe that there is no compile error anymore.

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/OverrideAnnotation.  You can just open it and run it.

Summary

In this exercise, you learned how to use a simple marker annotation called @Override to detect an unintentional overloading at the compile time.

Exercise 2: Single-member annotation

Many annotations will only require a single value to be associated with them.  Examine the Mutator.java file which contains the definition of a single membered annotation.  The goal of this exercise is to edit the AnnotatedClass.java  file and add a Mutator annotation to the setName method.

There are two approaches you can do this.  The first approach is to modify the code as following:

@Mutator(variable = “name”)
public void setName(String name)

The second approach is as following.  This is possible since this is a single member annotation we do not need to specify the name of the member to assign the value.  However, if you try to compile this code, you will get a compiler error.  Why?  The answer is that for single member annotations the identifier used for the member must be called value.  You will modify Mutator.java to correct this.

@Mutator(“name”)
public void setName(String name)

(2.1) Create and start a thread by implementing Runnable interface – start() method is not in the constructor

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in MutatorAnnotation as project name.
  • For Create Main Class field, type in MutatorAnnotation.
  • Click Finish.

  • Observe that MutatorAnnotation project appears and IDE generated RunnableThreadTest1.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MutatorAnnotation.java as shown in Code-2.11 below. The modification is to add a Mutator annotation to the setName method. we are using the first approach mentioned above. Study the code by paying special attention to the bold fonted parts.

public class MutatorAnnotation {

private String name;
private int id;

/**
*  Constructor
**/
public MutatorAnnotation() {
name = “Java Passion!”;
}

public String getName() {
return name;
}

@Mutator(variable = “name”)
public void setName(String name) {
this.name = name;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding vs. overloading
*  For exercise 1 change this method
**/
@Override
public boolean equals(Object otherName) {
String newName = (String) otherName;
int comparison = name.compareTo(newName);

return (comparison == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-2.11: MutatorAnnoation

3. Write Mutator.java.

  • Modify the IDE generated Mutator.java as shown below.
public @interface Mutator {
String variable();
}

Code-2.12: Mutator.java

4. Build (Compile) the project

  • Right click MutatorAnnotation project and select Build or Run.
  • Observe that compilation succeeds.

5. Modify the IDE generated MutatorAnnotation.java as shown in Code-2.13 below.

public class MutatorAnnotation {

private String name;
private int id;

/**
*  Constructor
**/
public MutatorAnnotation() {
name = “Java Passion!”;
}

public String getName() {
return name;
}

@Mutator(“name”)
public void setName(String name) {
this.name = name;
}

public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding vs. overloading
*  For exercise 1 change this method
**/
@Override
public boolean equals(Object otherName) {
String newName = (String) otherName;
int comparison = name.compareTo(newName);

return (comparison == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-2.13: Modified MutatorAnnotation.java

6. Observe the compile error. (Figure-2.13 and Figure-2.14 below) As mentioned earlier, for single member annotations, the identifier used for the member must be called value.

Compiling 2 source files to C:\handson2\development\javase5generics\samples(2)\MutatorAnnotation\build\classes
C:\handson2\development\javase5generics\samples(2)\MutatorAnnotation\src\MutatorAnnotation.java:18: cannot find symbol
symbol  : method value()
location: @interface Mutator
@Mutator(“name”)
1 error
BUILD FAILED (total time: 0 seconds)

Figure-2.14: Compile error


Figure-2.14: Compile error

6. Modify the Mutator.java as shown in Code-2.15 below.

public @interface Mutator {
String value();
}

Code-2.15: Modified Mutator.java

7. Build (Compile) the project

  • Right click MutatorAnnotation project and select Run or Build.
  • Observe that compilation succeeds.

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/MutatorAnnotation.  You can just open it and run it.

Summary

In this exercise, you learned how to add a single-member annotation using two diffferent approaches.

Exercise 3:Multiple member annotation

Annotations can be defined so that they have multiple values and, where required, default values can be provided.  In this exercise, you will learn how to define an annotation for accessor methods.  This annotation has both the name of the variable and the type of the variable defined as members of the annotation. You will also learn how to specify a default value – In order to specify a default value, add default “value” after the member name and before the semi-colon. . You will also learn how to modify the Accessor annotation so that the variableType has a default value of “String”.

 

(3.1) Multiple member annotation

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in MultipleMemberAnnotation as project name.
  • For Create Main Class field, type in MultipleMemberAnnotation.
  • Click Finish.

  • Observe that MultipleMemberAnnotation project appears and IDE generated MultipleMemberAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MultipleMemberAnnotation.java as shown in Code-3.11 below.  Study the code by paying special attention to the bold fonted parts.

 public class MultipleMemberAnnotation {
private String name;
private int id;

public MultipleMemberAnnotation() {
name = “Java Passion!”;
}

@Accessor(variableName = “name”)
public String getName() {
return name;
}

@Accessor(variableName = “name”, variableType = “int”)
public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

/**
*  Test of equals method overriding v. overloading
*  For exercise 1 change this method
**/
@Override
public boolean equals(Object otherName) {
String newName = (String) otherName;
int comparison = name.compareTo(newName);

return (comparison == 0);
}

public static void main(String[] args) {
// TODO code application logic here
}
}

Code-3.11: MultipleMemberAnnotation.java

3. Write Accessor.java as shown in Code-3.12 below.  This file defines an annotation for accessor methods.  (Accessor methods are the methods that accesses value of fields and takes the form of getXXX().)  This has both the name and the type of the variable defined as members of the annotation.  It also shows how to set a default value to one of its members.Study the code by paying special attention to the bold fonted parts.

/**
*  Annotation definition for an accessor method.  This shows the use of
*  multiple members.
**/
public @interface Accessor {
String variableName();
String variableType() default “String”;
}

Code-3.12: SimpleThread.java

4. Build and run the project

  • Right click MultipleMemberAnnotation project and select Build.
  • Observe that the compilation succeeds.
compile:
Building jar: C:\handson2\development\javase5generics\samples(2)\MultipleMemberAnnotation\dist\MultipleMemberAnnotation.jar
To run this application from the command line without Ant, try:
java -jar “C:\handson2\development\javase5generics\samples(2)\MultipleMemberAnnotation\dist\MultipleMemberAnnotation.jar”
jar:
BUILD SUCCESSFUL (total time: 0 seconds)

Figure-3.13: Result of running MultipleMemberAnnotation application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/MultipleMemberAnnotation.  You can just open it and run it.

Summary

In this exercise, you learned how to define an annotation that contains multiple members.

Exercise 4: Complex annotation types

Since an annotation is defined in the same way as an interface, it is possible to include types, in an annotation type definition, that they themselves are annotations.  In this exercise, you will learn how to define a complex annotation type called Name that contains a particular person’s first and last name and another annotation that uses  Name for the type of the value.

(4.1) Complex annotation types

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in ComplexAnnotation as project name.
  • For Create Main Class field, type in ComplexAnnotation.
  • Click Finish.

  • Observe that ComplexAnnotation project appears and IDE generated ComplexAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ComplexAnnotation.java as shown in Code-4.11 below.

@Reviewer(@Name(first = “James”, last = “Gosling”))
public class ComplexAnnotation {

public static void main(String[] args) {
// TODO code application logic here
}

}

Code-4.11: ComplexAnnotation.java

3. Write Name.java as shown in Code-4.12 below.

import java.lang.annotation.*;

public @interface Name {
String first();
String last();
}

Code-4.12: PrintStringsThread.java

4. Write Reviewer.java as shown in Code-4.13 below.  Study the code by paying special attention to the bold fonted parts.

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
public @interface Reviewer {
Name value();
}

Code-4.13: TwoStrings.java

5. Build and run the project

  • Right click ComplexAnnotation project and select Build.
  • Observe the compilationresult in the Output window. (Figure-4.14 below)
compile:
run:
BUILD SUCCESSFUL (total time: 0 seconds)

Figure-4.14: Result of running ComplexAnnotation application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/ComplexAnnotation.  You can just open it and run it.

Summary

In this exercise, you will learn how to define an annotation that uses another annotation.

 

Exercise 5: Meta annotation

Annotation types designed for annotating annotation type declarations are called meta-annotation types.  The package java.lang.annotation provides several of these.  The meta-annotation types can be used to restrict the annotation types they annotate.

  • @Target(ElementType.FIELD)
  • @Retention(RetentionPolicy.RUNTIME)

The Target meta-annotation is used to specify where the annotation is to be applied.  The possible choices include the following:

  • FIELD
  • TYPE (Class, interface or enum definition)
  • METHOD
  • PARAMETER
  • CONSTRUCTOR
  • LOCAL_VARIABLE
  • ANNOTATION_TYPE
  • PACKAGE

The Retention meta-annotation is used to specify how long an annotation is retained.  The possible choices include the following:

  • SOURCE: This annotation information is only retained in the source code and is not recorded in the generated class file.
  • CLASS:  This annotation is recorded in the class file by the compiler, but need not be retained by the virtual machine at runtime.  This is the default if @Retention is not specified.
  • RUNTIME:  Annotations are recorded in the class file by the compiler and retained by the virtual machine at runtime and can be read reflectively.

In this exercise, you learn how to use two meta-annotation types.

(5.1) Producer-Consumer without inter-thread communication

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in MetaAnnotation as project name.
  • For Create Main Class field, type in MetaAnnotation.
  • Click Finish.

  • Observe that MetaAnnotation project appears and IDE generated MetaAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated MetaAnnotation.java as shown in Code-5.11 below.

public class MetaAnnotation {

private String name;

@Exposed(“name”)
public void setName(String name) {
this.name = name;
}
public static void main(String[] args) {
// TODO code application logic here
}

}

Code-5.11: MetaAnnotation.java

3. Build and run the project

  • Right click MetaAnnotation project and select Build.
  • Observe the compile error. (Figure-5.12 below)
Compiling 2 source files to C:\handson2\development\javase5annotation\samples\MetaAnnotation\build\classes
C:\handson2\development\javase5annotation\samples\MetaAnnotation\src\MetaAnnotation.java:6: annotation type not applicable to this kind of declaration
@Exposed(“name”)
1 error
BUILD FAILED (total time: 0 seconds)

Figure-5.12: Compile error

4. Modify the MetaAnnotation.java as shown in Code-5.14 below.  The modification is to use the @Expose annotation to a field instead of a method as specified.

public class MetaAnnotation {

@Exposed(“name”)
private String name;

// @Exposed(“name”)
public void setName(String name) {
this.name = name;
}
public static void main(String[] args) {
// TODO code application logic here
}

}

Code-5.14: MetaAnnotation.java

4. Build and run the project

  • Right click MetaAnnotation project and select Build.
  • Observe that compilation succeeds.

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/MetaAnnotation.  You can just open it and run it.

5. For your own exercise, use annotation applicable to multiple places by using an array mechanism as shown in Code-5.15 below

@Target({ElementType.CONSTRUCTOR, ElementType.METHOD})

Code-5.15: Multiple application of TARGET

Summary

In this exercise, you learned how to use Target and Retention meta-annotation types.

 

Exercise 6: Reading annotations at runtime

As explained earlier, it is possible to specify that annotations are maintained in the classfile and made available via the runtime environment of the JVM.  To access the runtime information you need to use the reflection APIs which have been modified in J2SE 5.0 to include support for Metadata.

The Class class now has two additional methods:

  • getAnnotations() which returns an array of all annotations for the class
  • getAnnotation(Class c) which returns the information about the annotation of type c passed as a parameter.

The Method, Constructor and Field classes also have two new methods:

  • getAnnotation(Class c) which is the same as for Class
  • getDeclaredAnnotations() which returns an array of annotations declared for the Method, Constructor or Field.

(6.1) Schedule one-time task

1. Create a new NetBeans project

  • Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
  • Under Choose Project pane, select Java under Categories and Java Application under Projects.
  • Click Next.
  • Under Name and Location pane, for the Project Name field, type in RuntimeAnnotation as project name.
  • For Create Main Class field, type in RuntimeAnnotation.
  • Click Finish.

  • Observe that RuntimeAnnotation project appears and IDE generated RuntimeAnnotation.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated RuntimeAnnotation.java as shown in Code-6.11 below.  Study the code by paying special attention to the bold fonted parts.

import java.lang.annotation.*;

public class RuntimeAnnotation {

AnnotatedClass ac;

public RuntimeAnnotation() {
ac = new AnnotatedClass();
}

public void printAnnotations() {
Class c = ac.getClass();
Annotation[] annotations = c.getAnnotations();
int numberOfAnnotations = annotations.length;
System.out.println(“Class ” + c.getName() + ” has ” +
numberOfAnnotations + ” annotations”);

for (int i = 0 ; i < numberOfAnnotations; i++) {
System.out.println(“Annotation ” + i + “: ” + annotations[i] +
“, type” + annotations[i].annotationType().getName());
}
}

public static void main(String[] args) {
RuntimeAnnotation ar = new RuntimeAnnotation();
ar.printAnnotations();
}
}

Code-6.11: RuntimeAnnotation.java

3.  Write AnnotatedClass.java.

@Reviewer(@Name(first = “James”, last = “Gosling”))
public class AnnotatedClass {

private String name;
private int id;

public AnnotatedClass() {
name = “Java Passion!”;
}

@Accessor(variableName = “name”)
public String getName() {
return name;
}

@Mutator(variable =”name”)
public void setName(String name) {
this.name = name;
}

@Accessor(variableName = “name”, variableType = “int”)
public int getId() {
return id;
}

public void setId(int id) {
this.id = id;
}

public boolean equals(String otherName) {
int comparisson = name.compareTo(otherName);

return (comparisson == 0);
}
}

Code-6.12: AnnotatedClass.java.

4. Write Reviewer.java.  This is the same code you’ve written above.

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
public @interface Reviewer {
Name value();
}

Code-6.13: Reviewer.java

5. Write Name.java.  This is the same code you’ve written above.

import java.lang.annotation.*;

public @interface Name {
String first();
String last();
}

Code-6.14: Name.java

6. Write Mutator.java.  This is the same code you’ve written above.

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
public @interface Mutator {
String variable();
}

Code-6.15: Name.java

7. Write Accessor.java.  This is the same code you’ve written above.

import java.lang.annotation.*;

/**
*  Annotation definition for an accessor method.  This shows the use of
*  multiple members.
**/
@Retention(RetentionPolicy.RUNTIME)
public @interface Accessor {
String variableName();
String variableType() default “String”;
}

Code-6.16: Name.java

8. Build and run the project

  • Right click RuntimeAnnotation project and select Build.
  • Observe the result in the Output window. (Figure-6.17 below)
Class AnnotatedClass has 1 annotations
Annotation 0: @Reviewer(value=@Name(first=James, last=Gosling)), typeReviewer

Figure-6.17: Result of running UnRuntimeAnnotation application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javase5annotation/samples/RuntimeAnnotation.  You can just open it and run it.

Summary

In this exercise, you learned how to retrieve annotation information during runtime.

 


Homework exercise (for people who are taking Sang Shin’s “Java Programming online course”)

 

1. The homework exercise is to create MyOwnAnnotationExample project as following.

  • Create a new annotation called RequestForEnhancement as following
public @interface RequestForEnhancement {
int    id();
String synopsis();
String engineer() default “[unassigned]”;
String date();    default “[unimplemented]”;
}
  • Use the above annotation in your MyOwnAnnotationExample project to a method of your choice – in other words, you are using the above annotation to annotate a method of your choice –  by providing appropriate values.
  • Use Runtime annotation API to retrieve the values of the annotation (as you’ve done in Exercise 6 above) and display them.
2. Send the following files to javaprogramminghomework@sun.com with Subject as JavaIntro-javase5annotation.

  • Zip file of the the MyOwnAnnotationExample NetBeans project.  (Someone else should be able to open and run it as a NetBeans project.)  You can use your favorite zip utility or you can use “jar” utility that comes with JDK as following.
    • cd <parent directory that contains MyOwnAnnotationExample directory> (assuming you named your project as MyOwnAnnotationExample)
    • jar cvf MyOwnAnnotationExample.zip MyOwnAnnotationExample (MyOwnAnnotationExample directory should contain nbproject directory)
  • Captured output screen  – name it as JavaIntro-javase5annotation.gif orJavaIntro-javase5annotation.jpg (or JavaIntro-javase5annotation.<whatver graphics format>)
    • Any screen capture that shows that your program is working is good enough.  No cosmetic polishment is required.
  • If you decide to use different IDE other than NetBeans, the zip file should contain all the files that are needed for rebuilding the project – war file with necessary source files is OK.

 

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